Project Summary: The Antiphospholipid Antibody Syndrome (APS) is characterized by recurrent arterial and venous thrombosis in small and large vessels accompanied by persistently positive antiphospholipid antibody tests. This condition often affects young patients (particularly women of child-bearing age) and is associated with significant morbidity and mortality. Classically, studies of thrombosis have focused largely on platelets (arterial clots), red blood cells (venous clots) and the coagulation factors. However a growing body of data gathering over the past decade supports an intimate link between inflammation and thrombosis such that inflammation begets thrombosis and thrombosis amplifies the inflammatory response. Specifically, neutrophil activation and generation of neutrophil extracellular traps (NETs) have been shown to contribute to thrombosis in the setting of antiphospholipid antibodies. Further, NETs prime macrophages for inflammasome activation and release of cytokines and tissue factor implicating cross talk between members of the myeloid system to generate a proinflammatory and prothrombotic milieu. Presently, thrombosis risk is managed with lifelong anticoagulant therapy that is only modestly effective and associated with a high risk of bleeding, suggesting that alternate or novel approaches are necessary. These observations fuel the momentum for the view that therapies targeting the myeloid lineage may have potential impact on both arterial and venous thrombosis. However, the molecular mechanisms underlying myeloid cell activation in orchestrating this thrombo-inflammatory response is poorly understood. Our observations (both published and nascent) establish transcription factor KLF2 as a potent tonic repressor of myeloid cell activation and inflammation as well as an essential determinant of both arterial and venous thrombosis. Mechanistically, KLF2 is found to regulate two conserved ancient programs ? generation of neutrophil extracellular traps (NETs) and activation of the inflammasome. Specific to this proposal, we find that (i) APLA infusion increases both arterial and venous thrombosis in mice; (ii) APLA treatment leads to decreased KLF2 expression and inflammasome activation of macrophages and increased NET generation in neutrophils; (iii) myeloid-specific overexpression of KLF2 offers thromboprotection in the presence of APLA. Finally, the therapeutic potential of targeting KLF2 is revealed by our discovery that the anti-cancer agent bortezomib (BZ), at low non-toxic doses, induces myeloid KLF2 and confers potent anti-thrombotic effects (without altering hemostasis) in both arterial and venous beds that are KLF2-dependent. These observations provide the foundation for the central hypothesis that myeloid KLF2 is a central determinant of vascular thrombosis in APS that can be targeted for therapeutic gain. To explore this hypothesis, the following aims are proposed: (1) To investigate the effect of altering myeloid KLF2 on APLA-induced thrombosis in the macro- and microvasculature, (2) To elucidate the molecular mechanisms involved in KLF2?s ability to regulate NET formation and inflammasome activation in the context of APLA, and (3) To understand the molecular basis for the antithrombotic effect noted with BZ in the context of APLA treatment. These studies will reveal the molecular basis and functional consequences of altering myeloid KLF2 on thrombosis in the setting of APLA. Further, they will illuminate the importance of myeloid KLF2 in the antithrombotic/anti-inflammatory effects of BZ in this setting. Collectively, the results will not only provide deep biologic insights but may stimulate clinical studies evaluating the efficacy of BZ in APS with the potential to change standard of care for this devastating illness.